1. The Field of the Invention
This application relates to the demodulation of optical signals and, more particularly to demodulation of phase shift keyed signals.
2. The Relevant Technology
A phase shift keyed (PSK) optical signal may include a return-to-zero (RZ) signal having a series of relatively high intensity pulses separated by low intensity regions. A PSK optical signal may also be embodied as a non-return to zero (NRZ) signal. The phase difference between adjacent pulses encodes information. For example, a phase difference of π may encode a one, whereas a phase shift of zero or 2π encodes a zero bit. PSK signals have a distinct advantage in that both the zero bit and the one bit contain the same amount of optical energy, which enables a higher signal-to-noise ratio (SNR) at a demodulator as compared to encoding methods wherein a logical zero is encoded by a signal portion having a lower intensity than a logical one.
Demodulation of a PSK signal includes converting the phase information encoded in the pulses into amplitude modulation such that the data can be detected by means of a photodiode or other optical sensor. In a conventional demodulator, this is accomplished by means of a delay line interferometer (DLI), such as a Mach-Zehnder interferometer or Michelson interferometer. A DLI operates by dividing an input beam into first and second beams, such as by means of a beam splitter. The first and second beams travel along paths of different lengths and are then rejoined into an output beam, such as by means of another beam splitter. The difference in path length is chosen such that upon recombining, the first and second beams will constructively or destructively interfere with one another depending on the phase difference between adjacent pulses.
A conventional DLI-based demodulator has a number of significant disadvantages. In particular, the DLI-based demodulators tend to be large in size inasmuch as the difference in path length between the first and second paths must be large enough to introduce a one-bit delay. DLI-based demodulators are also somewhat expensive to manufacture because the delay between the first and second paths must be precisely tuned.